Trump-Biden transition: Attorney Sidney Powell back at White House Sunday

Biden on Thursday morning attended Mass at St. Joseph on the Brandywine Catholic Church, where the Bidens worship and where the remains of Biden's first wife, Neilia, and two children, Naomi and Beau, are buried in the church's adjacent cemetery.

It’s the 48th anniversary of the death of his wife and his infant daughter who were struck by a tractor-trailer as Neilia took the kids to pick out a Christmas tree. The tragedy occurred shortly after Biden was elected to his first term in the U.S. Senate.

Biden's son Hunter and late son Beau, ages 3 and 4 at the time, were also seriously injured. The Senator-elect ultimately took his oath of office from from his children's beside at a Delaware hospital.

The president-elect has no other public events scheduled for Friday.

With about a month to go until the inauguration, Biden is continuing to meet with transition advisers and build out what he's calling a Cabinet "full of firsts."

Biden officially announced new members of his climate team Thursday evening. New Mexico Rep. Deb Haaland was chosen for interior secretary and would be the first Native American Cabinet secretary, if confirmed. Michael Regan, current head of the North Carolina Department of Environmental Quality, was picked as EPA administrator and, if confirmed, would be the first Black man in the position. Michigan Gov. Jennifer Granholm rounded out the climate Cabinet posts with Biden nominating her as secretary of energy.

In addition to the Cabinet-level posts, Biden has also nominated Brenda Mallory to serve as chair of the Council on Environmental Quality. She would be the first African American to hold the position since its creation, if confirmed. Gina McCarthy was tapped to serve as the first-ever National Climate Advisor, heading up the newly formed White House Office of Domestic Climate Policy to “drive an “all of government” approach to combating climate change.” Along with McCarthy is Ali Zaidi, who helped draft the Obama administration's climate plan following the Paris Climate Agreement, to serve as deputy national climate adviser.

Trump, meanwhile, continues to challenge the vote behind closed doors, limiting his public appearances since the election. On Thursday evening, Trump praised incoming Alabama Sen. Tommy Tuberville, former Auburn football coach, after reports surfaced that the freshman lawmaker may join Alabama Rep. Mo Brooks’ Electoral College challenge, calling Tuberville “a great champion and man of courage.”

“More Republican Senators should follow his lead,” Trump tweeted.

Trump's pressure campaign on the GOP comes ahead of Congress convening a joint session to certify the election results on Jan. 6. The ceremonial proceeding, led every four years by the vice president, could represent Trump's last attempt to overturn his loss and disrupt the electoral process in protest.

Under federal law, a member can challenge the Electoral College results from any state if a member of the Senate backs the effort, forcing the House and Senate to separate for up to two hours of debate and a vote on whether to accept a slate of electors. A majority of both chambers would have to support the motion to successfully challenge a given slate of electors.

Trump, tagging Senate Majority Leader Mitch McConnell, on Friday tweeted at Republican Senators to "get tougher or you won’t have a Republican Party anymore."

Incoming White House senior adviser and director of the Office of Public Engagement Cedric Richmond has tested positive for COVID-19, Biden’s transition team said Thursday, noting that while Richmond did travel to Georgia for Biden’s event on Tuesday, he did not travel with Biden and was not in close contact with him as defined by the Centers for Disease Control and Prevention.

Incoming White House communications director Kate Bedingfield said in a statement Biden had a PCR test this morning that came back negative.

“Richmond was not in close contact, as defined by the CDC, with the President-elect. President-elect Biden underwent PCR testing for COVID-19 today and COVID-19 was not detected. On Tuesday, December 15th, Richmond traveled to Georgia for a campaign event with the Ossoff and Warnock campaigns. Neither the candidates nor any member of the Ossoff or Warnock campaign teams were in close contact with Richmond, nor were Mayor Keisha Lance Bottoms, Stacey Abrams or Nikema Williams, who also attended the Tuesday event,” the statement reads.

“Richmond’s interactions with the President-elect happened in open air, were masked and totaled less than 15 consecutive minutes, the CDC’s timeframe for close contact. Richmond traveled to Georgia on his own and not with the President-elect," it added.

Richmond developed symptoms on Wednesday and received two positive tests -- one rapid test on Wednesday and one PCR test Thursday. He will quarantine for 14 days and will produce two negative PCR tests before he returns to any in-person work in Congress or on the transition, according to Bedingfield.

News of Richmond's diagnosis could raise more questions about the timing of Biden's own vaccination, which is expected next week. Public health experts have said that Biden should be vaccinated as soon as possible for national security reasons.

"The sooner we can protect the incoming president, the better that would be," Dr. Leana Wen, a public health expert at George Washington University and a former Baltimore City health commissioner, told ABC News. "And the faster he can develop immunity, which is ultimately the goal."

The Louisiana Democrat is the 39th member of Congress to test positive for COVID-19, or be presumed positive, according to an ABC News tally. Several others have tested positive for COVID-19 antibodies.

-ABC News' Molly Nagle and Ben Siegel

Biden is expected to nominate Michael Regan, the leader of the North Carolina Department of Environmental Quality, as his pick for administrator of the Environmental Protection Agency, two sources familiar with the transition’s planning tell ABC News. 

If confirmed, Regan, 44, would be the first African American man to head the agency.

Biden has laid out an ambitious climate agenda that Regan would play a key role in carrying out, including addressing climate change, environmental inequality, and moving the country towards achieving a 100% clean energy economy and net-zero emissions no later than 2050. 

According to a source familiar with the president elect’s thinking, Biden was impressed by Regan’s leadership at the Department of Environmental Quality and his creation of the state's environmental justice and equity board to give communities disproportionately impacted by environmental inequality a role in the state’s environmental action. 

The source also pointed to Regan’s work holding polluters to account, particularly his role in reaching the largest coal ash cleanup settlement in U.S. history against Duke Energy.

-ABC News' Molly Nagle